Manufacturing method of opto-electric hybrid board and opto-electric hybrid board obtained thereby

ABSTRACT

A method of manufacturing an opto-electric hybrid board which is capable of reducing the number of steps for the manufacture of the opto-electric hybrid board and which achieves the reduction in thickness of the opto-electric hybrid board to be manufactured, and an opto-electric hybrid board obtained thereby. A plurality of protruding cores (optical interconnect lines)  3  are formed in a predetermined pattern. Thereafter, a thin metal film  4  is formed in grooves defined between adjacent ones of the cores  3.  Via-filling plating is performed on the thin metal film  4  to fill the above-mentioned grooves with a via-filling plated layer  6   a.  The plated layer  6   a  serves as electrical interconnect lines  6.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/028,692, filed Feb. 14, 2008, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing anopto-electric hybrid board in which an optical waveguide and electricalwiring are combined, and to an opto-electric hybrid board obtainedthereby.

2. Description of the Related Art

Recently, information and communications using light as a medium havecome into widespread use. An opto-electric hybrid board in which anoptical waveguide and electrical wiring are combined (see, for example,Japanese Patent Application Laid-Open No. 2001-7463) has accordinglybeen employed as a substrate for use in electronic devices forinformation and communications and the like.

In general, this opto-electric hybrid board is structured such that anelectrical wiring board including electrical interconnect lines(conductors) formed in a predetermined pattern and an optical waveguideincluding cores (optical interconnect lines) formed in a predeterminedpattern and serving as a passageway for light are stacked together. Anexample of the opto-electric hybrid board is shown in FIG. 3. Theopto-electric hybrid board B shown in FIG. 3 has a multi-layer structurehaving two layers in which an optical waveguide β is formed on anelectrical wiring board α. In the above-mentioned electrical wiringboard α, a plurality of electrical interconnect lines 96 are buried inan insulation layer 95 and are also supported by another insulationlayer 94 in that state. In the above-mentioned optical waveguide β, aplurality of cores 93 are buried in an over cladding layer 97 and arealso supported by an under cladding layer 92 in that state.

In the method of manufacturing the above-mentioned conventionalopto-electric hybrid board B, however, the process of producing theoptical waveguide β is performed after the process of producing theelectrical wiring board α, and each of the processes involves the needfor a multiplicity of steps. Thus, it takes a long period of time tomanufacture the opto-electric hybrid board B. For example, thepatterning of the electrical interconnect lines 96 in the electricalwiring board α involves the need for a large number of steps such as thesteps of patterning a resist through exposure, development and the like,plating other portions than the resist, and then removing theabove-mentioned resist. The patterning of the cores 93 in the opticalwaveguide β also involves the need for a large number of steps such asexposure, development and the like.

Additionally, the above-mentioned conventional opto-electric hybridboard B has the two-layer structure in which the optical waveguide β isstacked on top of the electrical wiring board α. Thus, theabove-mentioned conventional opto-electric hybrid board B isdisadvantageous in reducing the thickness thereof, and cannot respond torecent requests for the reduction in thickness.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide a method of manufacturing an opto-electric hybrid board which iscapable of reducing the number of steps for the manufacture of theopto-electric hybrid board and which achieves the reduction in thicknessof the opto-electric hybrid board to be manufactured, and anopto-electric hybrid board obtained thereby.

To accomplish the above-mentioned object, a first aspect of the presentinvention is intended for a method of manufacturing an opto-electronichybrid substrate, comprising the steps of: forming a plurality ofprotruding cores in a predetermined pattern on an under cladding layer;forming a thin metal film over the surface of the protruding cores and asurface portion of the under cladding layer except where the protrudingcores are formed; performing via-filling plating on the thin metal filmto fill grooves defined between adjacent ones of the protruding corescovered with the thin metal film with a via-filling plated layer;removing portions of the thin metal film and the plated layer which areformed on the top surface of the protruding cores to cause portions ofthe plated layer remaining in the grooves to serve as electricalinterconnect lines; and forming an over cladding layer so as to coverthe protruding cores and the electrical interconnect lines.

A second aspect of the present invention is intended for a nopto-electronic hybrid substrate comprising: a plurality of protrudingcores formed in a predetermined pattern on an under cladding layer; athin metal film formed over side surfaces of the protruding cores and asurface portion of the under cladding layer except where the protrudingcores are formed; electrical interconnect lines including portions of avia-filling plated layer buried in grooves defined between adjacent onesof the protruding cores covered with the thin metal film; and an overcladding layer formed so as to cover the cores and the electricalinterconnect lines.

In the method of manufacturing the opto-electric hybrid board accordingto the present invention, the plurality of protruding cores (opticalinterconnect lines) are formed in a predetermined pattern, andthereafter the grooves defined between adjacent ones of the cores arefilled with the via-filling plated layer which in turn serves as theelectrical interconnect lines (conductors). In other words, theelectrical interconnect lines are produced by using the grooves definedbetween the cores serving as one of the components of an opticalwaveguide and the like. Thus, the present invention eliminates the needto form a new pattern of the electrical interconnect lines andaccordingly reduces the number of steps for the manufacture of theopto-electric hybrid board. As a result, the present invention iscapable of reducing the time required for the manufacture of theopto-electric hybrid board to achieve improvements in productionefficiency. Further, since the pattern of the electrical interconnectlines is formed by using the pattern of the cores as mentioned above,the positioning accuracy of the cores and the electrical interconnectlines is automatically increased. Additionally, since theabove-mentioned electrical interconnect lines are formed by using thegrooves defined between the cores of the optical waveguide, themanufactured opto-electric hybrid board has what is called asingle-layer structure, and is significantly thinner than theconventional opto-electric hybrid board having a two-layer structure.

When the under cladding layer is formed on the metal base, only theopto-electric hybrid board is easily obtained by removing only theabove-mentioned metal base by etching after the opto-electric hybridboard is manufactured on the metal base.

In particular, when the metal base is a base made of stainless steel,the manufacture of the opto-electric hybrid board on the base isaccomplished with stability because the base made of stainless steel isexcellent in corrosion resistance and in dimensional stability.

The opto-electric hybrid board according to the present invention haswhat is called a single-layer structure because the electricalinterconnect lines are formed by using the grooves defined betweenadjacent ones of the cores as mentioned above. Thus, the above-mentionedcores and the electrical interconnect lines are formed at the samevertical position. This enables the opto-electric hybrid board accordingto the present invention to be significantly thinner than theconventional opto-electric hybrid board having a two-layer structure.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1( a) to 1(c) and FIGS. 2( a) to 2(c) are views schematicallyillustrating a method of manufacturing an opto-electric hybrid boardaccording to one preferred embodiment of the present invention; and

FIG. 3 is a view schematically illustrating a conventional opto-electrichybrid board.

DETAILED DESCRIPTION

Example embodiments according to the present invention will now bedescribed in detail with reference to the drawings.

FIGS. 1( a) to 1(c) and FIGS. 2( a) to 2(c) illustrate a preferredembodiment of a method of manufacturing an opto-electric hybrid boardaccording to the present invention. In this preferred embodiment, anunder cladding layer 2 made of an insulative material is formed on theentire top surface of a base 1 in the form of a stainless steel plateand the like as shown in FIG. 1( a). Subsequently, a plurality of cores(optical interconnect lines) 3 made of an insulative material axe formedto protrude in a predetermined pattern as shown in FIG. 1( b). Next, athin metal film 4 is formed over an outer surface (i.e., a top surface31 and opposite side surfaces 32) of the above-mentioned protrudingcores 3 and a surface portion of the under cladding layer 2 except wherethe cores 3 are formed by sputtering, electroless plating or the like asshown in FIG. 1( c). Then, via-filling plating is performed on the thinmetal film 4 to fill grooves 5 defined between adjacent ones of thecores 3 covered with the above-mentioned thin metal film 4 with avia-filling plated layer 6 a as shown in FIG. 2( a). Thereafter, asurface portion of the above-mentioned plated layer 6 a and the thinmetal film 4 on the top surface 31 of the cores 3 are removed by etchingas shown in FIG. 2( b). In this state, portions of the plated layer 6 aremaining in the grooves 5 defined between adjacent ones of the cores 3become electrical interconnect lines (conductors) 6. An over claddinglayer 7 made of an insulative material is formed to cover theabove-mentioned cores 3 and the above-mentioned electrical interconnectlines 6 as shown in FIG. 2( c). In this manner, an opto-electric hybridboard A having what is called a single layer structure in which thecores (optical interconnect lines) 3 and the electrical interconnectlines (conductors) 6 are arranged alternately is provided. Thisopto-electric hybrid board A, which is formed on the base 1, ismanufactured into a product after the base 1 in the form of a stainlesssteel plate is removed by treatment with an acid and the like ormanufactured into a product together with the base 1.

In this opto-electric hybrid board A, the under cladding layer 2, thecores 3 and the over cladding layer 7 constitute an optical waveguide,and the electrical interconnect lines 6 are formed in the grooves 5defined between the cores 3. The electrical interconnect lines 6, whichare formed by utilizing the insulating properties of the under claddinglayer 2, the cores 3 and the over cladding layer 7, are highly reliable.Also, the thin metal film 4 is formed on the opposite side surfaces 32of the cores 3, and may be used to function as a surface for reflectinglight beams. This further ensures the propagation of light beams.

The above will be described in further detail. The base 1 on which theabove-mentioned under cladding layer 2 is formed as shown in FIG. 1( a)has a flat shape, and a metal plate such as the above-mentionedstainless steel plate is used for the base 1. The material of the base1, however, is not limited to this. Examples of the material of the base1 used herein may include glass, quartz, silicon, synthetic resins andthe like. The thickness of the base 1 is, for example, in the range of20 μm (for a film-like base 1) to 5 mm (for a plate-like base 1).

The formation of the above-mentioned under cladding layer 2 as shown inFIG. 1( a) is carried out, for example, in a manner to be describedbelow. First, a varnish prepared by dissolving a photosensitive resin (ahighly insulative resin) conventionally known in the art in a solvent isapplied onto the above-mentioned base 1, and is then dried by a heatingtreatment to form a photosensitive resin layer. Next, the photosensitiveresin layer is exposed to irradiation light, and a heating treatment isperformed on the exposed photosensitive resin layer, whereby aphotoreaction is completed. This forms the above-mentionedphotosensitive resin layer into the under cladding layer 2. Thethickness of the under cladding layer 2 (the photosensitive resin layer)is typically in the range of 10 to 1000 μm.

For the formation of the above-mentioned under cladding layer 2, theapplication of the above-mentioned varnish is achieved, for example, bya spin coating method, a dipping method, a casting method, an injectionmethod, an ink jet method and the like. The subsequent heating treatmentis performed at 50° C. to 120° C. for 10 to 30 minutes. Examples of theirradiation light for the above-mentioned exposure used herein includevisible light, ultraviolet light, infrared light, X-rays, alpha rays,beta rays, gamma rays and the like. Preferably, ultraviolet light isused. This is because the use of ultraviolet light achieves irradiationwith large energy to provide a high rate of hardening, and anirradiation apparatus therefor is small in size and inexpensive toachieve the reduction in production costs. A light source of theultraviolet light may be, for example, a low-pressure mercury-vaporlamp, a high-pressure mercury-vapor lamp, an ultra-high-pressuremercury-vapor lamp and the like. The dose of the ultraviolet light istypically 10 to 10000 mJ/cm², preferably 50 to 3000 mJ/cm². Thesubsequent heating treatment is performed at 80° C. to 250° C.,preferably at 100° C. to 200° C., for 10 seconds to two hours,preferably for five minutes to one hour.

The patterning of the above-mentioned cores 3 with reference to FIG. 1(b), which is the subsequent step, is carried out, for example, in amanner to be described below. First, a photosensitive resin layer isformed on the above-mentioned under cladding layer 2 in a manner similarto the step of forming the above-mentioned under cladding layer 2. Next,the above-mentioned photosensitive resin layer is exposed to irradiationlight through an exposure mask formed with an opening patterncorresponding to the pattern of the cores 3 in a manner similar to thestep of forming the above-mentioned under cladding layer 2. Thereafter,a heating treatment is performed. Next, development is performed using adeveloping solution to dissolve away an unexposed portion of thephotosensitive resin layer, thereby forming the photosensitive resinlayer remaining on the under cladding layer 2 into the pattern of thecores 3. Thereafter, a heating treatment is performed to remove thedeveloping solution in the remaining photosensitive resin layer. Thus,the above-mentioned remaining photosensitive resin layer is formed intothe cores 3. The thickness or the cores 3 (the photosensitive resinlayer) is typically in the range of 10 to 100 μm. The width of the cores3 is typically in the range of 8 to 50 μm.

The material for the formation of the above-mentioned cores 3 usedherein is a material having a refractive index greater than that of thematerials for the formation of the above-mentioned under cladding layer2 and the over cladding layer 7 as shown in FIG. 2( c) to be describedlater. The adjustment of this refractive index may be made, for example,by adjusting the selection of the types of the materials for theformation of the above-mentioned under cladding layer 2, the cores 3 andthe over cladding layer 7 and the composition ratio thereof. Theabove-mentioned development employs, for example, an immersion method, aspray method, a puddle method and the like. Examples of the developingsolution used herein include an organic solvent, an organic solventcontaining an alkaline aqueous solution, and the like. The developingsolution and conditions for the development are selected as appropriatedepending on the composition of a photosensitive resin composition. Theheating treatment after the above-mentioned development is typicallyperformed at 80° C. to 120° C. for 10 to 30 minutes.

The above-mentioned thin metal film 4 to be formed subsequently withreference to FIG. 1( c) is a metal layer for use as a cathode during thevia-filling plating to be performed later with reference to FIG. 2( a).Examples of the metal material of the above-mentioned thin metal film 4include chromium, copper and the like. The thickness of theabove-mentioned thin metal film 4 is typically in the range of 600 to2600 Å.

The above-mentioned via-filling plating as shown in FIG. 2( a) isperformed by applying a voltage to a plating bath, with theabove-mentioned thin metal film 4 used as a cathode in the plating bath.This forms the plated layer 6 a obtained by the via-filling plating onthe surface of the above-mentioned thin metal film 4. In particular, amultiplicity of portions of the plated layer 6 a are formed in thegrooves 5 defined between adjacent ones of the cores 3 with reference toFIG. 1( c). Examples of the metal material of the plated layer 6 aformed by the above-mentioned via-filling plating include copper,nickel, gold, tin and the like.

Etching with reference to FIG. 2( b), which is the subsequent step is,carried out to remove portions of the thin metal film 4 which are formedon the top surface 31 of the above-mentioned cores 3 and portions of theplated layer 6 a shown in FIG. 2( a) which overlie the portions of thethin metal film 4. This is because, if those portions of the platedlayer 6 a and the thin metal film 4 are left unremoved, portions of theplated layer 6 a (the electrical interconnect lines 6) present in thegrooves 5 defined between adjacent ones of the cores 3 with reference toFIG. 1( c) are short-circuited. The above-mentioned etching is performedby immersion in an etchant. Examples of the etchant include aqueousferric chloride solutions, aqueous cupric chloride solutions, sulfuricacid and the like.

The formation of the above-mentioned over cladding layer 7 as shown inFIG. 2( c) after the above-mentioned etching is carried out in a mannersimilar to the formation of the above-mentioned under cladding layer 2.Specifically, a photosensitive resin layer is formed so as to cover theabove-mentioned cores 3 and the above-mentioned electrical interconnectlines 6. Thereafter, exposure to light and a heating treatment areperformed to form the above-mentioned photosensitive resin layer intothe over cladding layer 7. The thickness of the over cladding layer 7(the photosensitive resin layer) is typically in the range of 10 to 1000μm.

In this manner, the opto-electric hybrid board A is manufactured on thebase 1 as shown in FIG. 2( c). As described above, the opto-electrichybrid board A may be used while being provided on the base 1 or be usedafter the base 1 is separated therefrom or removed therefrom by etchingand the like.

In the above-mentioned opto-electric hybrid board A, the opticalwaveguide includes the plurality of cores 3, and the under claddinglayer 2 and over cladding layer 7 which hold the cores 3 therebetweenfrom below and from above. Also, the electrical interconnect lines 6 areformed between adjacent ones of the cores 3 in the optical waveguide, Inother words, the above-mentioned opto-electric hybrid board A has astructure such that the electrical interconnect lines (conductors) 6 areflush with (in the same layer as) the cores (optical interconnect lines)3 in the optical waveguide. Thus, the above-mentioned opto-electrichybrid board A is significantly thinner than the conventionalopto-electric hybrid board B (see FIG. 3) having the two-layerstructure. Further, since the thin metal film 4 is formed on theopposite side surfaces 32 of the cores 3 as stated above, the thin metalfilm 4 may be used to function as a surface for reflecting light beamspropagating through the cores 3. This further ensures the propagation ofthe light beams. Additionally, the electrical interconnect lines 6 areprevented from being short-circuited because the under cladding layer 2,the cores 3 and the over cladding layer 7 disposed around each of theelectrical interconnect lines 6 are insulators constituting theabove-mentioned optical waveguide.

The patterning of the cores 3 is achieved by exposing the photosensitiveresin layer to light and then developing the photosensitive resin layerin the above-mentioned preferred embodiment. This patterning, however,may be achieved by other methods, for example by cutting using a rotaryblade and the like. In this case, the material for the formation of thecores 3 is not limited to the photosensitive resin but may bethermosetting resins and the like.

The portions of the thin metal film 4 which are formed on the topsurface 31 of the cores 3 and the portions of the plated layer 6 a whichoverlie the portions of the thin metal film 4 are removed by etchingusing an etchant in the above-mentioned preferred embodiment. Thisremoval, however, may be achieved by other methods, for example by aphysical method such as polishing and the like.

In the above-mentioned preferred embodiment, the formation of the undercladding layer 2 and the over cladding layer 7 uses the photosensitiveresin as the materials thereof, and is achieved by exposure and thelike. However, other materials and other methods may be used. As anexample, the formation of the under cladding layer 2 and the overcladding layer 7 may use a thermosetting resin such as polyimide resinand epoxy resin as the materials thereof, and may be achieved byapplying a varnish prepared by dissolving the thermosetting resin in asolvent and then performing a heating treatment (typically at 300° C. to400° C. for 60 to 180 minutes) to set the varnish or by other methods. Aresin film may be used as the under cladding layer 2 and the overcladding layer 7.

Next, an example of the present invention will be described. It shouldbe noted that the present invention is not limited to the example.

EXAMPLE

Material for Formation of Under Cladding Layer and Over Cladding Layer

A material for formation of an under cladding layer and an over claddinglayer was prepared by mixing 35 parts by weight of bisphenoxyethanolfluorene glycidyl ether (component A) represented by the followinggeneral formula (1), 40 parts by weight of3′,4′-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate which is analicyclic epoxy resin (CELLOXIDE 2021P manufactured by Daicel ChemicalIndustries, Ltd.) (Component B), 25 parts by weight of(3′,4′-epoxycyclohexane)methyl-3′,4′-epoxycyclohexyl-carboxylate(CELLOXIDE2081 manufactured by Daicel Chemical Industries, Ltd.) (Component C),and one part by weight of a 50% propione carbonate solution of4,4-bis[di(β-hydroxyethoxy)phenylsulfinio]phenyl-sulfide-bis-hexafluoroantimonate(component D).

wherein R₁ to R₆ are hydrogen atoms, and n=1.Material for Formation of Cores

A material for formation of cores was prepared by dissolving 70 parts byweight of the aforementioned component A, 30 parts by weight of1,3,3-tris{4-[2-(3-oxetanyl)]butoxyphenyl}butane and one part by weightof the aforementioned component D in 28 parts by weight of ethyllactate.

Manufacture of Opto-Electric Hybrid Board

The material for the formation of the above-mentioned under claddinglayer was applied to the surface of a base made of stainless steel(having a thickness of 20 μm) by a spin coating method, and was thendried by a heating treatment at 100° C. for 15 minutes. Then, exposureby the use of irradiation with ultraviolet light at 2000 mJ/cm² wasperformed through a photomask (exposure mask) formed with a desiredopening pattern. Next, a heating treatment was performed at 100° C. for15 minutes to form the under cladding layer. The thickness of this undercladding layer was 25 μm when measured with a contact-type filmthickness meter. The refractive index of this under cladding layer at awavelength of 830 nm was 1.542.

Next, the material for the formation of the above-mentioned cores wasapplied to the surface of the above-mentioned under cladding layer by aspin coating method, and was then dried by a heating treatment at 100°C. for 15 minutes. Next, a photomask formed with an opening patternidentical in shape with a core pattern was placed over the resultingcore material. Then, exposure by the use of irradiation with ultravioletlight at 4000 mJ/cm² was performed by a contact exposure method fromover the mask. Thereafter, a heating treatment was performed at 120° C.for 30 minutes. Next, development was carried out using an aqueoussolution of γ-butyrolactone to dissolve away an unexposed portion.Thereafter, a heating treatment was performed at 120° C. for 30 minutesto form the protruding cores. When measured with an SEM (electronmicroscope), the dimensions of the cores in cross section were 50 μm inwidth×50 μm in height, and a spacing between adjacent ones of the coreswas 50 μm. The refractive index of the cores at a wavelength of 830 nmwas 1.588.

Next, a thin metal film (having a thickness of 1500 Å) made of an alloyof chromium and copper was formed by sputtering over the surface of theprotruding cores and a surface portion of the under cladding layerexcept where the cores were formed. Then, via-filling plating wasperformed on the surface of the above-mentioned thin metal film to forma plated layer made of copper, thereby filling grooves defined betweenadjacent ones of the cores with the above-mentioned plated layer.Thereafter, the surface portion of the above-mentioned plated layer andthe thin metal film on the top surface of the cores were removed untilthe top surface of the cores was uncovered by immersion in an etchant ofan aqueous ferric chloride solution. Then, portions of the plated layerremaining in the grooves were used to function as electricalinterconnect lines.

Next, the over cladding layer was formed so as to cover theabove-mentioned cores and the above-mentioned electrical interconnectlines in a manner similar to the formation of the above-mentioned undercladding layer. The thickness of the over cladding layer was 25 μm whenmeasured with a contact-type film thickness meter. The refractive indexof the over cladding layer at a wavelength of 830 nm was 1.542.

In this manner, the opto-electric hybrid board in which the cores(optical interconnect lines) and the electrical interconnect lines(conductors) were arranged alternately at the same vertical position andin which the under cladding layer and the over cladding layer weredisposed under and over the alternating cores and electricalinterconnect lines was manufactured on the base.

Although a specific form of embodiment of the instant invention has beendescribed above and illustrated in the accompanying drawings in order tobe more clearly understood, the above description is made by way ofexample and not as a limitation to the scope of the instant invention.It is contemplated that various modifications apparent to one ofordinary skill in the art could be made without departing from the scopeof the invention which is to be determined by the following claims.

1. A method of manufacturing an opto-electronic hybrid substrate,comprising the steps of: forming a plurality of protruding cores in apredetermined pattern on an under cladding layer; forming a thin metalfilm over the surface of the protruding cores and a surface portion ofthe under cladding layer except where the protruding cores are formed;performing via-filling plating on the thin metal film to fill groovesdefined between adjacent ones of the protruding cores covered with thethin metal film with a via-filling plated layer; removing portions ofthe thin metal film and the plated layer which are formed on the topsurface of the protruding cores to cause portions of the plated layerremaining in the grooves to serve as electrical interconnect lines; andforming an over cladding layer so as to cover the protruding cores andthe electrical interconnect lines.
 2. The method according to claim 1,wherein the under cladding layer is formed on a metal base.
 3. Themethod according to claim 2, wherein the metal base is a base made ofstainless steel.
 4. An opto-electronic hybrid substrate comprising: aplurality of protruding cores formed in a predetermined pattern on anunder cladding layer; a thin metal film formed over side surfaces of theprotruding cores and a surface portion of the under cladding layerexcept where the protruding cores are formed; electrical interconnectlines including portions of a via-filling plated layer buried in groovesdefined between adjacent ones of the protruding cores covered with thethin metal film; and an over cladding layer formed so as to cover thecores and the electrical interconnect lines.